William is 3 times as old as Luis and is also 20 years older than Luis. How old is Luis?
Answer: We can use the given information to write down two equations that describe the ages of William and Luis. Let William's current age be $w$ and Luis's current age be $l$ $w = 3l$ $w = l + 20$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $l$ , and both of our equations have $w$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3l$ $-$ $ (l + 20)$ which combines the information about $l$ from both of our original equations. Solving for $l$ , we get: $2 l = 20$ $l = 10$.